Quantisation of relativistic Hamilton function

When a charged particle is moving under the influence of electromagnetic potentials (the vector potential A and the scalar potential 4 ) then the relativistic expression for the energy (the relativistic Hamilton function) is 

For a slow-moving particle (1 v/c) the expansion of the square root in powers of nfmQc is possible 

The quantum mechanical equation is obtained by substituting the energy and momentum for corresponding operators 

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